1. Field of the Invention
The present invention relates to microwave antenna feed systems, and more particularly to a dual frequency band feed system for a parabolic antenna or the like.
2. Description of the Prior Art
The use of geostationary satellites for providing communications for video, data, and audio circuits has become widespread. Presently, two frequency bands are being used; the C-band which covers 3.7-4.2 GHz and the Ku band in the frequency range from 9-15 GHz. With the prevalence of receive only television ground stations, high efficiency feed systems have been developed. As the higher frequency Ku band becomes more widely used, many ground stations presently operating in the lower frequency C-band will require antennas to receive the higher frequency signals. The most economical approach is to utilize the same parabolic reflector. If one feed system is placed on the boresight of the antenna, it would be necessary for the feed system for the other band to be displaced a number of beamwidths from the boresight. While such arrangement could produce maximum efficiency on one band, the efficiency on the other band would be reduced. Thus, there is a need for a feed system which allows both high band and low band feeds to be at the boresight of a parabolic reflector to maintain maximum efficiency for each band.
While dual and multiple feeds are known in the prior art, none are suitable for the present application. The following U.S. Pat. Nos. disclose various types of multi-frequency feeds: 3,665,481 Lowe et al; 3,369,197 Giger et al; 3,864,687 Walters et al; 4,345,257 Brunner; 4,420,756 Amata et al; and 4,442,437 Chu et al.
Lowe et al disclose a plural coaxial horn feed for use on a spacecraft which transmits at two frequencies and receives tracking signals at a third frequency. A conical horn is used on the high frequency feed which blocks a portion of the lower band feed as well as degrading the VSWR. The feed is designed for circular polarization and utilizes the TE.sub.11 and TM.sub.01 modes. For use with the geostationary satellites, dual linear polarization is required. Giger et al teach a waveguide with separate coaxial cavities with selective mode separation within the same frequency band. Several modes are fed. Various diameter guides are utilized and are coupled together in sequence. In Amata et al, a multimode tracking antenna feed system is described in which the low frequency feed utilizes the TE.sub.11 mode and the high frequency feed utilizes the TM.sub.01 and TE.sub.11 modes. Chu et al disclose a dual mode feedhorn which allows the propagation of TE.sub.11 modes in both the high and low frequency bands. Walters et al teach a wide band multimode antenna which has a plurality of coaxial, independently fed radiating horns.